A frequency comb is a spectrum consisting of a series of discrete, equally spaced elements. The frequency domain representation of a perfect frequency comb is a series of delta functions spaced as f(n)=f0+nfr, where n is an integer, fr is the comb tooth spacing, and f0 is the carrier offset frequency (f0<fr). Such a frequency comb facilitates a direct link from radio frequency standards to optical frequencies and may be useful with respect to a number of different applications, such as spectroscopy, coherent optical communications, and microwave photonics.
A variety of different methods for optical frequency comb generation have been proposed. The existing technologies for the generation of optical frequency combs can be generally classified into three types of generators: Optical frequency comb generators based on femtosecond mode-locked laser; Optical frequency comb generators based on electro-optic modulator (EOM); and Optical frequency comb generators based on 3rd-order nonlinear Kerr effect in micro-cavities. In operation of the optical frequency comb generators based on the conventional femtosecond mode locked lasers, the frequency spacing of the generated optical frequency comb is quite limited by the laser cavity length (i.e., limited to approximately 10 GHz). Although the frequency spacing can be extended to tens of GHz in operation of the optical frequency comb generators based on EOM, the frequency spacing is nevertheless limited by the bandwidth of the EOMs and RF oscillators. In operation of the prior optical frequency comb generators based on nonlinear Kerr effect of micro-cavities, the frequency spacing of the generated comb can be expanded to hundreds of GHz, however these optical frequency comb generators typically require strict wavelength matching and thermal stabilization when external direct laser pumping or one fiber loop laser cavity is used.
As can be appreciated from the foregoing, prior optical frequency generators show non-ideal performance of the small frequency spacing and narrow spectrum bandwidth. Such traditional optical frequency comb generation techniques are not well suited to meet needs for larger frequency spacing and broader spectrum bandwidth. Moreover, most of the traditional techniques for optical frequency comb generation lack flexibility, for which the frequency spacing and the shape of the generated comb cannot been changed easily.